Two main processes have been used for wood pulping: mechanical pulping and chemical pulping. Mechanical pulping primarily uses mechanical energy to separate pulp fibers from wood without a substantial removal of lignin. As a result, the yield of mechanical pulping is high, typically in the range of 85-98%. The produced fiber pulps generally have high bulk and stiffness properties. However, mechanical pulping consumes a high level of operational energy, and the mechanical pulps often have poor strength.
In order to reduce the required energy level and improve fiber strength, other process options have been used in a combination with mechanical energy. Thermomechanical pulping (TMP) grinds wood chips under steam at high pressures and temperatures. Chemi-thermomechanical pulping (CTMP) uses chemicals to break up wood chips prior to a mechanical pulping. The CTMP pulping has somewhat lower yield than mechanical pulping, but it provides pulp fibers with a slightly improved strength. Sodium sulfite has been the main chemical used for CTMP pulping. Within the past 10 years, the industry has begun to use alkaline hydrogen peroxide as an impregnation chemical and as a chemical directly applied to a high consistency refiner treatment for CTMP pulping. This pulping process, known as alkaline peroxide mechanical pulping (APMP), provides fiber pulps with enhanced brightness and improved strength compared to the traditional CTMP pulping. Additionally, recent breakthroughs in the APMP pulping process have been associated with a reduction of the required refining energy through an application of a secondary, low consistency refining system and an enhancement of barrier screening technology to selectively retain rejects while allowing the desirable fibers to pass through to a paper machine.
Chemical wood pulping is a process to separate pulp fibers from lignin by employing mainly chemical and thermal energy. Normally, lignin represents about 20-35% of the dry wood mass. When the majority of the lignin is substantially removed, the pulping provides approximately a 45-53% pulp yield.
Chemical pulping reacts wood chips with chemicals under pressure and temperature to remove lignin that binds pulp fibers together. Chemical pulping is categorized based on the chemicals used into kraft, soda, and sulfite. Alkaline pulping (AP) uses an alkaline solution of sodium hydroxide with sodium sulfide (kraft process) or without sodium sulfide (soda process). Acid pulping uses a solution of sulfurous acid buffered with a bisulfite of sodium, magnesium, calcium, or ammonia (sulfite process). Chemical pulping provides pulp fibers with, compared to mechanical pulping, improved strength due to a lesser degree of fiber degradation and enhanced bleachability due to lignin removal.
In the chemical process, wood is “cooked” with chemicals in a digester so that a certain degree of lignin is removed. A kappa number is used to indicate the level of the remaining lignin. The pulping parameters are, to a large degree, able to be modified to achieve the same kappa number. For example, a shorter pulping time may be compensated for by a higher temperature and/or a higher alkali charge in order to produce pulps with the same kappa number.
Kraft pulping has typically been divided into two major end uses: unbleached pulps and bleachable grade pulps. For unbleached softwood pulps, pulping is typically carried out to a kappa number range of about 65-105. For bleachable grade softwood kraft pulps, pulping is typically carried out to a kappa number of less than 30. For bleachable grade hardwood kraft pulps, pulping is typically carried out to a kappa number of less than 20.
For bleachable grade pulps, kraft pulping usually generates about 1-3 weight % of undercooked fiber bundles and about 97-99 weight % of liberated pulp fibers. The undercooked, non-fiberized materials are commonly known as rejects, and the fiberized materials are known as accepts pulp. Rejects are separated from accepts pulp by a multiple stage screening process. Rejects are usually disposed of in a sewer, recycled back to the digester, or thickened and burned. In a few circumstances, rejects are collected and recooked in the digester. However, using this prior technology, drawbacks exist from recooking the rejects which include an extremely low fiber yield, a potential increase in the level of pulp dirt, and a decrease in pulp brightness (poorer bleachability).
Modern screen rooms are typically designed to remove about 1-2 weight % of rejects from a chemical pulping process. If a mill experiences cooking difficulties and accidentally undercooks the pulp, the amount of rejects increases exponentially. Modern bleachable grade kraft pulp screen rooms are not physically designed to process pulps with greater than about 5% by weight of rejects. When the level of rejects increases to slightly above 4-5% by weight, either the screen room plugs up and shuts down the pulp mill, or the screen room is bypassed and the pulp is dumped onto the ground or into an off quality tank and disposed of or gradually blended back into the process. Therefore, bleachable grade kraft pulps are conventionally cooked to relatively low kappa numbers (20-30 for softwoods and 12-20 for hardwoods) to maintain a low level of rejects and good bleachability.
There has been a continuing effort to increase the yield of a chemical pulping process, while maintaining the chemical pulp performance such as high strength. In 2004-2007, the U.S. Department of Energy's Agenda 20/20 program sponsored several research projects to achieve this manufacturing breakthrough endeavor. The Agenda 20/20 program, American Forest and Products Association (AF&PA), and the U.S. Department of Energy jointly published a book in 2006 that define one of the performance goals for breakthrough manufacturing technologies would be “Produce equivalent/better fiber at 5% to 10% higher yield”. Target pulp yield increases of 5-10% are considered to be revolutionary to the pulp producing industry. To date, the Agenda 20/20 funded projects have achieved, at best, a 2-5% pulp yield increase. These developed technologies include a double oxygen treatment of high kappa pulps, a use of green liquor pretreatment prior to pulping, and a modification of pulping chemicals and additives used for pulping. However, all other known attempts to achieve a breakthrough of 5-10% yield increase have failed. Other known chemical pulping modifications to increase pulp yield include a use of digester additives such as anthraquinone, polysulfide, penetrant or various combinations of these materials. Again in all instances, only 1-5% yield increase over a traditional kraft pulping process has been realized. Additionally, the modified chemical pulping process often provides fiber pulps with lower tear strength.
Accordingly, there is a need for a novel pulping process with a breakthrough yield (i.e., 5-10% increase) that is economically feasible. Furthermore, the pulp fibers from such pulping process should exhibit equivalent or enhance physical properties to those of the conventional, lower yield pulping processes.
Two critical performances for paperboard packaging are stiffness and bulk. The packaging industry strives for paper/paperboard with high stiffness at a lowest basis weight possible in order to reduce the weight of paper/paperboard needed to achieve a desired stiffness and, therefore, to reduce raw material cost.
One conventional approach to enhance the board stiffness is through using single-ply paperboard with a higher basis weight. However, a single-ply paperboard with an increased basis weight is economically undesirable because of a higher raw material cost and higher shipping cost for the packaging articles made of such board.
Another conventional practice is to use multiply paperboard having at least one middle or interior ply designed for high bulk performance with top and bottom plies designed for stiffness. U.S. Pat. No. 6,068,732 teaches a method of producing a multiply paperboard with an improved stiffness. Softwood is chemically pulped, and the resulting fiber pulps are screened into a short fiber fraction and a long fiber fraction. The outer plies of paperboard are made of the softwood long fiber fraction. The center ply of paperboard is formed from a mixture of the softwood short fiber fraction and chemically pulped hardwood fibers. The paperboard has about 12-15% increase in Taber stiffness. PCT Patent Application No. 2006/084883 discloses a multiply paperboard having a first ply to provide good surface properties and strength and a second ply comprising hardwood CTMP (chemi-thermomechanical) pulps to provide bulkiness and stiffness.
Multiply paperboards are commonly prepared from one or more aqueous slurries of cellulosic fibers concurrently or sequentially laid onto a moving screen. Production of multiply board requires additional processing steps and equipments (e.g., headbox and/or fourdrinier wire) to the single ply boards. Conventionally, a first ply is formed by dispensing the aqueous slurry of cellulosic fibers onto a long horizontal moving screen (fourdrinier wire). Water is drained from the slurry through the fourdrinier wire, and additional plies are successively laid on the first and dewatered in similar manner. Alternatively, additional plies may be formed by means of smaller secondary fourdrinier wires situated above the primary wire with additional aqueous slurries of cellulosic fibers deposited on each smaller secondary fourdrinier wire. Dewatering of the additional plies laid down on the secondary fourdrinier wires is accomplished by drainage through the wires usually with the aid of vacuum boxes associated with each fourdrinier machine. The formed additional plies are successively transferred onto the first and succeeding plies to build up a multiply mat. After each transfer, consolidation of the plies must be provided to bond the plies into a consolidated multiply board. Good adhesion between each ply is critical to the performance of multiply board, leading to an additional factor that may deteriorate board properties. The plies must be bonded together well enough to resist shear stress when under load and provide Z-direction fiber bond strength within and between plies to resist splitting during converting and end use. However, a multiply paperboard with an increased basis weight is economically undesirable because of a higher production cost and higher shipping cost for the packaging articles made of such board.
Therefore, there is a need for paperboard having an enhanced stiffness at a lower basis weight that is more economical than conventional single-ply and multiply paperboards.
Unbleached products are commonly produced using either (1) substantial amounts of unbleached, low kappa number hardwood kraft pulps, or (2) blends of high yield unbleached pine and unbleached, low kappa number hardwood pulps. Saturating kraft pulp grades are typically made with (1) unbleached hardwood pulps, or (2) unbleached hardwood pulps with small amounts, about 10 weight percent, of cut up high yield unbleached pine pulps. A key measure of the performance of saturating kraft pulps is saturability and resin pick up. Other product grades are a blend of unbleached, low kappa hardwood and unbleached high yield pine to produce board packaging grades. Stiffness and printability are key performance parameters for these types of boards. Finally, several linerboard products are produced in a multilayer format with high yield pine on the bottom layer and unbleached, low kappa hardwood in the top layer. STFI stiffness and smoothness are key quality concerns for these products.